Non-oriented electromagnetic steel sheet having excellent magnetic properties after stress relief annealing

ABSTRACT

A finish annealed non-oriented electromagnetic steel sheet includes about 0.01 wt % or less of C, greater than about 1.0 wt % and at most about 3.5 wt % of Si, at least about 0.6 wt % and at most about 3.0 wt % of Al, at least about 0.1 wt % and at most about 2.0 wt % of Mn, at least about 2 ppm and at most about 80 ppm of one or more rare earth metals (REM), a maximum content of Ti and Zr being about 15 ppm and 80 ppm, respectively, wherein oxygen on the surface layer of the steel sheet is 1.0 g/m 2  or less. Since the non-oriented electromagnetic steel sheet has desirable mechanical properties resulting from the increased amounts of Si and Al, a high magnetic flux density can be maintained without sacrificing a punching property as well as very low iron loss can be obtained even after stress relief annealing.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a non-oriented electromagnetic steelsheet having excellent magnetic properties after stress relief annealingand a method of manufacturing the same.

2. Description of the Related Art

Non-oriented electromagnetic steel sheets have been used as the ironcore materials of motors, transformers, and the like. It is desirable tolower the iron loss of the non-oriented electromagnetic steel sheet inorder to increase the energy efficiency of these devices.

Recently, it has become especially important to make the motors moreefficient. Accordingly, it is desired to improve the magnetic propertiesof the non-oriented electromagnetic steel sheet, in particular, toincrease its magnetic flux density and lower its iron loss. Also, therotor unit thickness of a DC brushless motor, for example, is reduced upto about 5 mm by embedding a permanent magnet into a rotor. Accordingly,adequate mechanical strength, which has not been important inconventional small motors, is also required for the non-orientedelectromagnetic steel sheet, in addition to the magnetic properties.That is, there is required an electromagnetic steel having excellentmagnetic properties and adequate mechanical strength as a material forsmall motors of high efficiency.

As a means for reducing the iron loss of the non-orientedelectromagnetic steel sheet, there is available a method of optimizing agrain size and a method of improving the specific resistance of thesteel sheet. That is, it is well known that the iron loss is minimizedby the grain size of about 150-200 μm, that the addition of Si or Al iseffective to improve a specific resistance, and that mechanicalproperties depend on Si and Al in steel.

On the other hand, it is also well known that a problem arises in that asaturation magnetic flux density is reduced and the punching property ofsteel sheet is deteriorated when the content of Si or Al is increased.In particular, the punching property is a very important property forthe non-oriented electromagnetic steel sheet. Non-orientedelectromagnetic steel sheet is often used by users after it is punchedto a prescribed shape and then subjected to stress relief annealing.Since the punched shape is complicated and requires accuracy, a precisepunching property is required for the non-oriented electromagnetic steelsheet. The punching property is deteriorated by the increase of thehardness and grain size of the electromagnetic steel sheet. The increaseof the hardness and grain size results from an increase in the alloyingcomponents of the steel sheet or scales formed on the surface of thesteel sheet. For example, when Si exceeds 1.0 wt % or when the grainsize of a finished steel sheet exceeds 40 μm, a problem arises in thatthe punching property is deteriorated.

Accordingly, the recent demand for higher motor efficiency and adequatemechanical strength requires a material having an excellentgrain-growing property after stress relief annealing, which thereby hasa high magnetic flux density and very low final iron loss withoutsacrificing its punching property.

This need can be met by sufficiently increasing the Si and Al contentthereby to coarsen the crystal grains. In particular, it is preferred toincrease the content of Al because it has less effect on increasedhardness. Also, it is preferred to coarsen crystal grains because itreduces the iron loss after stress relief annealing. More specifically,although Si and Al have the same degree of specific resistanceincreasing effect, the Al content is increased because the effect of Alper unit weight on the increase of hardness is about one half that ofSi. On the other hand, although increasing the stress relief annealingtemperature is effective to coarsen crystal grains, the grain growingproperty must be improved in a relatively low stress relief temperatureregion of about 750° C. at the highest, which is employed in practicedue to cost considerations.

Japanese Unexamined Patent Publication No. 8-3699 discloses a low Sinon-oriented electromagnetic steel sheet in which a Si component islowered to 1.0 wt % or less to obtain an excellent growing propertyafter stress relief annealing and a low final iron loss. The graingrowing property of the non-oriented electromagnetic steel sheet isgreatly improved by adding REM (rare earth metals) to the steel andhighly purifying the steel during the steel-making process. High purityis accomplished by suppressing the contents of Ti and Zr, which areelements contained in a trace amount. Precipitates which deteriorategrain growth are controlled by REM-addition or purification. Accordingto the publication, this works remarkably well; however, due to the lowSi content, the problem arises that the mechanical strength isinsufficient for some locations where the steel sheet is used and ironloss of the sheet is insufficient to meet the need for a greaterreduction in iron loss of the cores.

Japanese Examined Patent Publication No. 61-4892 seeks to improvemagnetic properties by increasing the Al content. However, althoughmechanical properties were improved by the increase of only the Alcontent, the magnetic properties were greatly altered. In particular, alow loss product could not be obtained stably after stress reliefannealing as described later. It has been found by the inventors thatthe above problem is caused by nitriding during the stress reliefannealing.

In Japanese Unexamined Patent Publication No. 8-296007, it is disclosedthat the deterioration of the magnetic properties of a steel sheetcontaining a large content of Al is suppressed by controlling Ccontained in an insulating film, because the deterioration is caused bynitriding in stress relief annealing. According to the publication,however, although the change in magnetic properties is reduced, thedegree of reduction remains insufficient and it is necessary to suppressthe change altogether.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide anon-oriented electromagnetic steel sheet having not only excellentmagnetic properties after stress relief annealing but also excellentmechanical properties, and to propose an advantageous manufacturingmethod for the non-oriented electromagnetic steel sheet.

The inventors have investigated the levels to which Al and Si should beset, on the premise that REM is added and a steel sheet is highlypurified in order to more greatly reduce iron loss after stress reliefannealing and to improve mechanical properties. As a result, theinventors have confirmed that an increase in Al content reduces ironloss without significantly deteriorating a punching property, and isaccordingly suitable for the improvement of magnetic properties.However, a serious problem had still arisen in that the magneticproperties were still altered after stress relief annealing due to theincrease in an Al content. As a result of a diligent study to solve theabove problem, the inventors have newly found that it is very important,in a non-oriented electromagnetic steel sheet whose iron loss isintended to be reduced after stress relief annealing, to control surfacescales produced during finish annealing in addition to making thecomponents and precipitates in a steel adequate, in order tosimultaneously achieve good mechanical properties and the stableimprovement of iron loss after stress relief annealing by an increase inSi and Al contents.

The present invention results from the above discovery.

According to the present invention, a non-oriented electromagnetic steelsheet comprises at most about 0.01 wt % of C, greater than 1.0 wt % andat most about 3.5 wt % of Si, at least about 0.6 wt % and at most about3.0 wt % of Al, at least about 0.1 wt % and at most about 2.0 wt % ofMn, at least about 2 ppm and at most about 80 ppm of REM, with Ti and Zrbeing suppressed to at most about 15 ppm and 80 ppm, respectively,wherein the amount of oxygen on the metal surface layer of the steelsheet is 1.0 g/m² or less after finish annealing.

It is preferable that the non-oriented electromagnetic steel sheetfurther comprises at least about 0.002 wt % and at most about 0.1 wt %of at least one of Sb and Sn.

It is advantageous for the stable improvement of magnetic propertiesthat the non-oriented electromagnetic steel sheet further comprises S, Oand N suppressed to 20 ppm or less, 15 ppm or less and 30 ppm or less,respectively, and that the ratio of the number of REM-containinginclusions coupled with nitride to the number of REM-containinginclusions having a diameter of at least about 1 μm in the steel sheetis 40% or more.

The non-oriented electromagnetic steel sheet is manufactured by thesteps of hot rolling and cold rolling a steel slab comprising at mostabout 0.01 wt % of C, greater than 1.0 wt % and at most about 3.5 wt %of Si, at least about 0.6 wt % and at most about 3.0 wt % of Al, atleast about 0.1 wt % and at most about 2.0 wt % of Mn, at least about 2ppm and at most about 80 ppm of REM, with Ti and Zr being suppressed toat most about 15 ppm and 80 ppm, respectively, and subjecting the thusrolled steel sheet to finish annealing by adjusting at least one of adew point and a gas atmosphere to thereby control the amount of oxygenon the metal surface layer of the steel sheet to 1.0 g/m² or less.

It is preferable that the steel slab used in this method furthercomprises at least about 0.002 and at most about 0.1 wt % of at leastone of Sb and Sn.

In these manufacturing methods, it is advantageous to the stableimprovement of magnetic properties that when a molten steel is made, REMis added after S and O in the molten steel is adjusted to 40 ppm or lessand 25 ppm or less, respectively, to thereby suppress S and O to 20 ppmor less and 15 ppm or less, respectively, as well as N is adjusted to 30ppm or less so that the ratio of the number of REM-containing inclusionscoupled with nitride to the number of REM-containing inclusions having adiameter of at least about 1 μm in the steel sheet is 40% or more afterfinish annealing.

In the manufacturing methods of the present invention, it is preferablethat the hot-rolled sheet is annealed for 40 seconds or less at 700° C.or more to 1150° C. or less after the hot-rolling and that the finishannealing is performed in a soaking time of 15 seconds or less at 750°C. or more to 900° C. or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual view showing REM-containing inclusions coupledwith nitride.

DESCRIPTION OF PREFERRED EMBODIMENTS

Embodiments of the present invention will be specifically describedbelow.

At first, the reasons for the disclosed contents of the respectivecomponents will be described.

C: 0.01 wt % or less

Since C deteriorates magnetic properties by the precipitation ofcarbide, it should be limited to 0.01 wt % or less.

Si: greater than about 1.0 wt % and at most about 3.5 wt %

Si is a component useful to reduce iron loss by increasing a specificresistance. When the content of Si is 1.0 wt % or less, reduction of theiron loss is insufficient and mechanical properties do not improve. Sothe content of Si should be greater than 1.0 wt %. By the increase ofthe Si content, the iron loss can be reduced by increasing the specificresistance and mechanical strength, for example, tensile strength andyield stress can be increased. However, when the Si content isexcessively increased, hardness is excessively increased. Consequently,a punching property is deteriorated. Furthermore a cold rolling becomesdifficult in manufacture. Therefore, it is necessary to set the Sicontent to less than or equal to 3.5 wt %. In particular, it is morepreferable to set the Si content to more at least about 1.0 wt % and atmost about 2.5 wt %.

Mn: 0.1-2.0 wt %

Since Mn acts to fix S as coarse MnS, Mn should be contained in anamount of 0.1 wt % or more and preferably 0.5 wt % or more. On the otherhand, since an excessive increase in the additive amount of Mndeteriorates a magnetic flux density, Mn should be contained in anamount of 2.0 wt % or less and preferably in an amount of 1.5 wt % orless.

Al: at least about 0.6 wt % and at most about 3.0 wt %

Al is an important component in the present invention. Al is aneffective element to reduce iron loss in the same degree as Si byincreasing the specific resistance of a product. Moreover, Al has asmall hardening capability (a hardness increasing amount per unitweight), about one half that of Si. Consequently, Al is an effectiveelement to suppress the hardening of a product and can improve magneticproperties without sacrificing mechanical properties. Furthermore, sinceAl is an element which forms precipitates by nitriding, an increase inthe additive amount of Al can suppress the dispersion of fine AlN in amanufacturing process and improve a grain growing property in subsequentrecrystallization annealing and stress relief annealing. Iron loss isreduced by grain growth. The simultaneous addition of Al and REM canimprove the grain growing property greatly as described below.Furthermore, Al has an effect for increasing crystal grains which have[100] orientation. The [100] orientation is preferred in terms ofmagnetic properties.

When Al is contained in an amount less than 0.6 wt %, sufficientmechanical properties cannot be obtained. When Al is contained in anamount greater than 3.5 wt %, a problem arises in the punching property,the cold rolling ability in manufacture, and the excessive deteriorationof the magnetic flux density. Therefore, the content of Al is set to atleast about 0.6 wt %, at most about 3.5 wt %, and preferably to 0.6 to1.5 wt %.

REM: 2-80 ppm

The addition of one or more kinds of rare earth elements (REM) in atotal amount of 2-80 ppm can avoid the adverse affect of Zr on thegrowth of grains in stress relief annealing. Zr is inevitably containedin an amount of 5-80 ppm in a steel making process executed on anindustrial scale. Furthermore, it has been confirmed that when Al isadded in a large amount, the grain growing property can be greatlyimproved by the further addition of REM. It is predicted that thisbecause the addition of REM changes the precipitated state of otherprecipitates. Although the reason is not apparent, it is believed thatREM oxide and REM sulfide act as a nucleating site when fineprecipitates such as Zr nitride or Al nitride precipitate. These effectsare insufficient when REM is contained in an amount less than 2 ppm,whereas the excessive addition of REM increases the inclusions formed byREM and a problem arises in that the grain growth is obstructed by REMinclusions themselves. Accordingly, REM is contained in an amount of 80ppm or less and preferably in an amount less than 50 ppm.

Ti: 15 ppm or less

Ti is set to 15 ppm or less because it greatly deteriorates iron loss bylowering the grain growing property in stress relief annealing executedat a low temperature even if it is contained in a very small amount.When Ti is set to 10 ppm or less, even better reduction of iron loss canbe obtained. It should be noted that the addition of Ti alone is not soeffective even if its content is set to 15 ppm or less. On the otherhand, the simultaneous addition of Ti and REM can improve the graingrowing property during low-temperature stress relief annealing.Although the reason is not apparent, it is believed that REM oxide andREM sulfide act as a nucleus creating site when fine precipitates ofsuch as Ti and the like precipitate.

Zr: 80 ppm or less

It is preferable to reduce the content of Zr as much as possible becauseZr deteriorates the grain growing property in the low temperature stressrelief annealing even if it is contained in a very small amount.However, it requires a remarkable increase of production cost to stablyset Zr to 5 ppm or less on an industrial scale. Thus, in the presentinvention Zr is preferably set to 5-80 ppm. Zr becomes harmless in thisrange which can be stably achieved industrially by the addition of REM.More specifically, a remarkable effect for reducing iron loss can beobtained by setting Zr content to 80 ppm or less in combination with theaddition of REM. Although the reason is not apparent, it is believedthat REM oxide and REM sulfide act as a nucleating site when fineprecipitates such as Zr nitride and Al nitride are formed.

Furthermore, the grain growing property can be improved by controllingthe form of the REM inclusions in a steel containing Al in an amount of0.6 wt % or more as described below. More specifically, the graingrowing property can be further improved by keeping the ratio of thenumber of REM-containing inclusions coupled with nitride to the numberof REM-containing inclusions having a diameter of at least about 1 μm at40% or more, together with an O content of 15 ppm or less, a S contentof 20 ppm or less, and a N content of 30 ppm or less. The REM-containinginclusions coupled with nitride are, for example, made by REM-containingoxide and REM-containing sulfide coupled with nitride such as AlN andthe like. This example is shown in FIG. 1. And the ratio of the numberof this kind of the inclusions to the number of the entireREM-containing inclusions is regulated as to the REM-containinginclusions having the diameter of at least about 1 μm.

The reason is not apparent why the grain growing property is improved bythe control of the form of the REM-containing inclusions and thelimitation of the O content, the S content and the N content in thesteel. However, it is believed that the grain growing property isimproved by the reduction of the oxide, nitride, sulfide and thecomposite of them which form the precipitates in the steel to theirpossible limits and growing the nitride precipitate as large REMinclusions which do not affect the grain growing property in the stressrelief annealing.

Furthermore, the amount of oxide on the surface of a high Al steel sheetcan be reduced by including one or both of Sb and Sn in a total amountat least about 0.002 wt % and at most about 0.1 wt %. Sb and Sn areelements for suppressing surface oxidation. In particular, when at leastone of Sb and Sn is added to the steel which contains basic componentsof the present invention, the surface oxidation can be more effectivelysuppressed so that a material having excellent properties can be stablyobtained. When Sn and Sb are contained in an amount less than 0.002 wt%, they are not effective to suppress the surface oxidation. On theother hand, when the Sn and Sb content exceeds 0.1 wt %, significantamounts of Sn and Sb are segregated to the grain boundary, whichobstructs their movement. Consequently, the grain growing property inthe stress relief annealing is deteriorated. Accordingly, the Sb and Sncontent is set to 0.1 wt % or less.

Although the components other than the above components are notparticularly limited in the present invention, it is preferable to limitthe contents of the following components.

P: 0.2 wt % or less

P can be added to improve the punching property. When, however, Pcontent exceeds 0.2 wt %, a cold rolling property is deteriorated.Therefore, it is preferable to add P in an amount of 0.2 wt % or less.

S: 0.01 wt % or less

S forms MnS precipitate together with Mn. MnS obstructs the movement ofa magnetic domain wall and the growth of grains, so the magneticproperties deteriorate. It is preferable to set the S content to 0.01 wt% or less, the less the better.

N: 0.01 wt % or less

N generates nitrides which obstruct the movement of the magnetic domainwall and the growth of the grains, so the magnetic propertiesdeteriorate. It is preferable also to set the N content to 0.01 wt % orless, the less the better.

O: 50 ppm or less

When O is contained in an amount of more than 50 ppm, O generates oxideswhich obstruct the movement of the magnetic domain wall and the growthof the grains, so the magnetic properties deteriorate. It is preferableto set an O content to 50 ppm or less, the less the better.

Cu: 0.05 wt % or less

When Cu is contained in an amount exceeding 0.05 wt %, Cu generates Cusulfide which obstructs the movement of the magnetic domain wall and thegrowth of the grains, so the magnetic properties deteriorate. It ispreferable to set a Cu content to 0.05 wt % or less, the less thebetter.

Nb: 0.005 wt % or less

Nb generates Nb carbide or Nb nitride which obstructs the movement ofthe magnetic domain wall and the growth of the grains, so the magneticproperties deteriorate. It is preferable to set a Nb content to 0.005 wt% or less, the less the better.

B: 0.0005 wt % or less

B forms BN which obstructs the movement of the magnetic domain wall andthe growth of the grains, so the magnetic properties deteriorate. It ispreferable to set a B content to 0.0005 wt % or less, the less thebetter.

It is preferable to reduce elements such as V, Mo, Cr and the like whichare concerned in the formation of the precipitates in steel such asoxide, nitride, sulfide and the like as much as possible among theinevitable impurities in combination with the reduction of the amountsof O, N and C.

In the present invention, the amount of oxygen on the metal surfacelayer of a steel sheet should be set to 1.0 g/m² or less on thecompletion of finish annealing after the above-mentioned components areadjusted as described above.

The inventors carefully considered the effect of Al as described above.When the inventors reduced iron loss by increasing the amount of Al, theinventors confirmed that prescribed iron loss could be obtainedaccording to conventional knowledge. However, it was difficult to obtainstable magnetic properties because the magnetic properties were deviatedafter the stress relief annealing. At first, the inventors supposed thatthe deviation of magnetic properties was caused by the effect of impureelements and studied the effect of them. But studying the effect ofimpurity elements did not account for the deviation. After variousexaminations, the inventors made the new discovery that a nitridingphenomenon occurred during the stress relief annealing and was a mainfactor of the deviation. This nitriding phenomenon depends upon surfacescales which were produced when the sheet was subjected to the finishannealing in a steel containing Al in a large amount. More specifically,it was discovered that when the amount of oxygen on the metal surfacelayer exceeded 1.0 g/m², nitriding was remarkably accelerated during thestress relief annealing which was executed after the finish annealing.As a result, the scales caused a magnetism deteriorating phenomenon andthey were liable to be made by the increased amount of Al in the finishannealing. Although the reason why the magnetism deterioratingphenomenon was caused was not apparent, it was believed that the form ofsurface scales affected the nitriding phenomenon as the amount of oxygenincreased.

The conditions for manufacturing a non-oriented electromagnetic steelsheet according to the present invention and also the reasons for thoseconditions will now be described.

At first, a molten steel is manufactured according to a conventionalsteel-making process such as a converter-degassing process or the like.The molten steel is made into a slab by a continuous casting process ora casting-ingot-making process. In order to set the ratio of the numberof REM-containing inclusions coupled with nitride to the number ofREM-containing inclusions having a diameter of at least about 1 μm to40% or more, O content in the steel to 15 ppm or less, a S content to 20ppm or less and a N content to 30 ppm or less, which are the conditionsto further improve a grain growing property in stress relief annealing,the following treatment is preferred:

(1) the O content in the molten steel is reduced to 25 ppm or less bysufficiently performing degassing—Al deoxidizing;

(2) the S content is adjusted to 40 ppm or less by adding adesulfurizing agent; and

(3) thereafter the S content is suppressed to 20 ppm or less, the Ocontent is suppressed to 15 ppm or less and the N content is adjusted to30 ppm or less by adding REM.

Although REMs are elements that form oxides and sulfides, it isespecially likely to couple with the oxygen in a steel. Thus, the Ocontent remaining in the steel must be sufficiently reduced that enoughdesulfurizing can occur by formation of REM-sulfide. More specifically,the coupling of REM with O can be reduced sufficiently if the O contentis set to 25 ppm or less in the steel before the REM is added.Consequently, the sulfide can be effectively created by REM. Since thethus created REM sulfide and oxide partly float at the time when REM isadded, the O content in the steel is finally reduced to 15 ppm or less.Note, since the S content should be finally reduced to 20 ppm or lessthrough the desulfurization performed by the sulfide created by REM, theS content should be reduced to about 40 ppm before the addition of theREM. A desulfurizing agent such as an ordinary flux or the like can beused. On the other hand, although it is preferable to set the N contentto about 40 ppm or less before the addition of REM, it is sufficientthat it is finally adjusted to 30 ppm or less. Note, when the Al contentin the steel is increased according to the present invention, thereoccurs a deoxidizing effect by Al, whereby the amount of oxygen in thesteel is reduced before the addition of REM.

Another object of setting the O and S contents to 25 ppm or less and 40ppm or less, respectively, before the addition of REM is to set theratio of the number of the REM-containing inclusions coupled withnitride to the number of REM-containing inclusions having a diameter ofat least about 1 μm in the steel to 40% or more. It is not apparent whythe ratio of the number of the REM-containing inclusions coupled withnitride to the number of REM-containing inclusions having a diameter ofat least about 1 μm can be set to 40% or more by the adjustment of the Sand O contents before the addition of REM; however, it is believed thatthe N content is relatively increased by the reduction of the totalamounts of O and S in the steel which are coupled with REM before theaddition of REM so that the ratio of inclusions which are combined withTi nitride and Zr nitride is increased. The Ti nitride and the Zrnitride are created when they are coupled with nitride during the steelbeing solidified and cooled. The reason why the grain growing propertyis improved by setting the ratio of the number of the REM-containinginclusions coupled with nitrogen to the number of REM-containinginclusions having a diameter of at least about 1 μm is as describedabove.

Subsequently the slab is hot rolled. The slab can be either reheated andthen hot rolled or directly hot rolled without reheating. When aparticularly high magnetic flux density is needed, the aggregatestructure can be improved by coarsening crystal grains of a hot-rolledsheet by self-annealing performed during hot-rolled sheet coiling afterhot rolling. Either box annealing (for example, 850° C.×1 hour) orcontinuous annealing (for example, 950° C.×2 minutes) is suitable as thehot-rolled sheet annealing.

The hot-rolled sheet is annealed continuously in a short time from theview of cost reduction and productivity improvement. But a high magneticflux density cannot be obtained by conventional techniques when ahot-rolled sheet is annealed for a short time such as a soaking time of30 seconds, because the crystal grains of the hot-rolled sheet are notsufficiently coarsened. However, since the grain growing property isimproved by the present invention, a high magnetic flux density can beobtained even if the hot-rolled sheet is annealed for a relatively shorttime. The hot-rolled steel sheet must conventionally be annealed for atleast about 5 minutes to obtain an excellent magnetic flux density. Onthe other hand, since the steel composition of the present invention isgreatly improved in the grain growing property, the hot-rolled sheet canbe annealed in a time shortened to 40 seconds or less. At that time,when the annealing temperature is less than about 750° C., the effect ofcoarsening crystal grains of the hot-rolled sheet by the annealing issmall, whereas when the annealing temperature exceeds about 1150° C.,the annealing becomes uneconomical. Thus, it is preferable to set theannealing temperature from 750° C. or more to 1150° C. or less.

Next, the hot-rolled sheet is made as a product either by cold rollingthe hot-rolled sheet once so that it has the thickness of the productand then finish annealing the cold-rolled sheet, or by cold rolling thehot-rolled sheet twice with intermediate annealing performedtherebetween and then finish annealing the cold-rolled sheet.

The finish annealing should be performed so that the amount of oxygen onthe metal surface layer of the steel sheet is controlled to 1.0 g/m² orless after the completion of the finish annealing as described above.The amount of the oxygen on the metal surface layer of the steel sheetis controlled by adjusting at least one of the dew point and the gasatmosphere during the finish annealing. Incidentally, it is advantageousto set an oxygen potential represented by P(H₂O)/P(H₂) to 0.7 or less byadjusting, for example, either or both of the dew point and the gasatmosphere so that the finish annealing can be performed at 600° C. ormore and 1100° C. or less. P(H₂O) represents a partial pressure of H₂Ogas and P(H₂) represents a partial pressure of H₂ gas. The amount ofoxygen on the metal surface layer of the steel sheet is mainlycontrolled by the atmosphere such as the dew point, a gas compositionand the like, although it also depends on an annealing time, and it issufficient to regulate the annealing time from the viewpoint ofproductivity. Since the steel composition according to the presentinvention is excellent in the grain growing property, it is possible toperform the finish annealing in a short time. That is, the finishannealing can be performed in 15 seconds or less in the temperaturerange from 750° C. or more and 900° C. or less. Needless to say, theamount of oxygen on the metal surface layer should be adjusted to 1.0g/m² also at the time. As to the other finish annealing conditions, anyof the conditions for manufacturing a non-oriented electromagnetic steelsheet excellent in iron loss after stress relief annealing isapplicable.

It is possible to form an insulating film on the surface of the steelsheet and perform skin pass rolling of 2-10% after the finish annealingby known methods. The same effect can be obtained even if theseprocesses are added.

EXAMPLES Example 1

A slab containing the components shown in Table 1 was made by continuouscasting after it was subjected to a converter-degassing process. Theresulting slab was reheated and then hot rolled to form a hot-rolledsteel sheet. Thereafter, the hot-rolled steel sheet was annealed for 25seconds at 950° C. and rolled to form a cold-rolled steel sheet having athickness of 0.5 mm by cold rolling including pickling. Subsequently,the cold-rolled steel sheet was finish annealed for 14 seconds at 800°C. in various annealing atmospheres and an insulating film was formed.In the finish annealing, a dew point was adjusted in a mixed gasatmosphere containing 35% H₂ and 65% N₂so that the amount of oxygen onthe surface layer of the steel sheet was controlled. Thereafter, stressrelief annealing was performed for 2 hours at 750° C. in a dry nitrogenatmosphere, and magnetic properties were measured by a 25 cm Epsteinmethod. Table 1 shows the result of the analysis of components, magneticproperties measurement and punching property evaluation, respectively.The punching property is evaluated by the burrs of a punched piece edgeafter punching to a shape having a 30 mm diameter with an SKD metal moldis performed 200,000 times. When a punched product had a burr exceeding20 μm, the product was evaluated as an insufficiently punched productand provided with a mark x.

Mechanical strength was evaluated by the yield strength y_(p) of aproduct sheet. A product having yield strength y_(p) exceeding 300 N/mm²was marked with o which meant acceptable, whereas a product having yieldstrength y_(p) less than 300 N/mm² was marked with x which meantunacceptable.

This is also applicable to the respective examples which will bedescribed later.

TABLE 1 Evalu- ation Mecha- Evaluation nical Punchi pro- No. AmountW_(15/50) ng perty an- Component (after finish) of (after proper Ypneal- C REM Ti Zr Sb Sn oxygen stress ty (N/mm²) ing (ppm) Si Al Mn P(ppm) (ppm) (ppm) Sb Sn (g/m²) relief) * ** Reference 1 28 0.6 0.68 0.30.03 5 10 20 tr tr 0.1 2.9  ∘ X Comparative 2 25 0.8 0.79 0.31 0.02 1110 21 tr tr 0.1 2.85 ∘ X Comparative 3 23 1.21 0.25 0.28 0.03 5 11 32 trtr 0.1 2.88 ∘ X Comparative 4 22 1.22 0.75 0.29 0.02 11 10 21 tr tr 0.12.59 ∘ ∘ Inventive 5 24 1.21 2.28 0.32 0.01 4 12 19 tr tr 0.09 2.43 ∘ ∘Inventive 6 20 1.85 2.21 0.33 0.03 8 9 19 tr tr 0.15 2.28 ∘ ∘ Inventive7 29 2.2 1.59 0.31 0.04 10 9 26 tr tr 0.15 2.28 ∘ ∘ Inventive 8 31 2.23.1 0.32 0.02 8 9 22 tr tr 0.19 2.25 X ∘ Comparative 9 33 3.8 1.8 0.360.04 12 9 21 tr tr — Cold — — Comparative rolling Example impossible 1032 1.23 0.7 0.81 0.05 15 9 23 tr tr 0.17 2.49 ∘ ∘ Inventive 11 32 1.261.6 2.3 0.03 11 9 21 tr tr 0.16 2.26 X ∘ Comparative 12 122 1.2 1.6 0.320.04 8 9 22 tr tr 0.16 3.13 ∘ ∘ Comparative 13 22 1.2 1.6 0.31 0.03 7 621 tr tr 0.1 2.38 ∘ ∘ Inventive 14 24 1.21 1.58 0.31 0.03 6 7 21 tr tr0.13 2.39 ∘ ∘ Inventive 15 28 1.19 1.57 0.28 0.04 8 8 60 tr tr 0.11 2.42∘ ∘ Inventive 16 26 1.2 1.59 0.29 0.02 8 7 90 tr tr 0.13 2.99 ∘ ∘Comparative 17 30 1.18 1.55 0.32 0.02 1 or 6 30 tr tr 0.14 2.91 ∘ ∘Comparative 18 36 1.22 1.52 0.33 0.03 22 6 30 tr tr 0.16 2.39 ∘ ∘Inventive 19 25 1.21 1.61 0.31 0.05 71 6 30 tr tr 0.19 2.41 ∘ ∘Inventive 20 26 1.19 1.63 0.32 0.01 100 6 30 tr tr 0.2 2.91 ∘ ∘Comparative 21 28 1.2 1.59 0.33 0.03 7 13 23 tr tr 0.3 2.4  ∘ ∘Inventive 22 25 1.21 1.58 0.35 0.04 6 20 21 tr tr 0.25 2.89 ∘ ∘Comparative 23 29 1.22 1.54 0.34 0.01 1 or 10 24 tr tr 0.24 2.79 ∘ ∘Comparative 24 23 1.22 1.58 0.32 0.03 6 10 25 tr tr 0.8 2.39 ∘ ∘Inventive 25 22 1.21 1.59 0.33 0.04 6 10 26 tr tr 1.3 2.79 ∘ ∘Comparative 26 21 1.21 1.56 0.34 0.02 11 10 21 0.05 tr 0.08 2.33 ∘ ∘Inventive 27 22 1.15 1.58 0.32 0.03 12 9 22 tr 0.05 0.06 2.34 ∘ ∘Inventive 28 24 1.16 1.59 0.3 0.02 13 8 26 0.05 0.05 0.06 2.31 ∘ ∘Inventive ∘: good X: poor **∘: Y_(p) ≧ 300 N/mm² X: Y_(p) < 300 N/mm²

It is found from Table 1 that the comparative examples Nos. 1 and 2which contain Si in a small amount and the comparative example No. 3which contained Al in a small amount could not provide low iron loss,the comparative example No. 9 which contained Si in an amount exceeding3.5 wt % was broken during cold rolling, the comparative example No. 8which contained Al in an amount exceeding 3.1 wt % was insufficientlypunched, the comparative example No. 11 which contained Mn in an amountexceeding 2.0 wt % was also insufficiently punched, and the comparativeexample No. 12 which contained C in an amount exceeding 0.01 wt % haddeteriorated magnetic properties.

The comparative examples Nos. 17 and 23 which contained no REM hadmagnetic properties of a low level even if they contained Ti and Zr inamounts of 15 ppm or less and 80 ppm or less, respectively, whereas whenthey contained REM and the Ti and Zr were contained therein in amountsof 15 ppm or less and 80 ppm or less, respectively, a remarkable ironloss reducing effect could be obtained. Note, the comparative examplesNos. 22 and 16 contained Ti and Zr in amounts exceeding 15 ppm and 80ppm, respectively, and had deteriorated magnetic properties.

Next, the inventive examples Nos. 13, 24 and the comparative example No.25 whose amounts of oxygen on the metal surface layer of the steel sheetexceeded 1.0 g/m² had deteriorated magnetic properties after the stressrelief annealing. Furthermore, the inventive examples Nos. 26-28 hadimproved mechanical properties because an amount of scales was reducedby the addition of Sb and/or Sn.

Example 2

A slab containing the components shown in Table 2 was made by continuouscasting after it was subjected to a converter-degassing process. CaO wasadded after the addition of Al. In the above processes, CaO was addedinto molten steel after Al was added, then REM was added, and the moltensteel was stirred. Subsequently, the resulting slab was reheated andthen hot rolled to form a hot-rolled steel sheet. Thereafter, thehot-rolled steel sheet was annealed in 20 seconds at 950° C. and rolledto form a cold-rolled steel sheet having a thickness of 0.5 mm by coldrolling including pickling. Subsequently, the cold-rolled steel sheetwas finish annealed for 9 seconds at 800° C. and made into products. Inthe finish annealing, a dew point was adjusted in a mixed gas atmospherecontaining 35% H₂ and 65% N₂so that the amount of oxygen on the surfacelayer of the steel sheet was controlled. After the components of theresulting products were analyzed and the inclusions of the products wereexamined, test pieces were sampled and subjected to stress reliefanealing for 2 hours and 750° C. in a dry nitrogen atmosphere, andmagnetic properties were measured. The result of the investigation issummarized in Table 2.

TABLE 2 (*) Com- posi- tion ratio Component of before the Evaluation REMaddition of Amount W_(15/50) Composition (after finish annealing) inclu-REM of (after C (**) REM Ti Zr S O N sions S O oxygen stress Refer- No.ppm Si Al Mn p ppm ppm ppm (ppm) (ppm) (ppm) (%) (ppm) (ppm) (g/m²)relief) ence 1 23 1.23 1.61 0.31 0.03 9 6 10 25 18 33 26 50 18 0.1 2.38Inven- tive product 2 21 1.22 1.59 0.29 0.03 10 6 10 15 8 21 46 36 160.1 2.25 Inven- tive product 3 22 1.21 1.6 0.3 0.03 11 6 11 16 8 23 3056 18 0.1 2.35 Inven- tive product 4 23 1.22 1.61 0.31 0.03 10 6 12 17 925 33 38 27 0.1 2.34 Inven- tive product 5 22 1.21 1.58 0.3 0.03 11 5 1545 17 22 23 62 26 0.1 2.36 Inven- tive product 6 23 1.22 1.6 0.29 0.02 96 16 19 9 33 45 37 19 0.1 2.35 Inven- tive product 7 23 1.21 1.6 0.290.02 9 5 16 19 19 35 45 36 18 0.1 2.36 Inven- tive product (*)Composition ratio of REM inclusions: the ratio of the number ofREM-containing inclusions coupled with nitride which are occupied in theREM-containing inclusions having a diameter of at least about 1 μm (**)Inevitably mixed component

It is found from Table 2 that an inventive product No. 2 which was moreexcellent in iron loss after stress relief annealing could be obtainedby setting the amounts of S, O and N to 20, 15 and 30 ppm, respectivelyand setting the ratio of the number of nitride REM inclusions containedin REM inclusions to 40% or more. It can be found that the ratio of thenumber of the nitride REM inclusions was set to 40% or more by settingthe amount of S to 40 ppm or less and the amount of O to 25 ppm or lessbefore the addition of the REM.

Example 3

A slab containing the components shown in Table 3 was made by continuouscasting after it was subjected to a converter-degassing process. Theresulting slab was reheated and then hot rolled to form a hot-rolledsteel sheet. Thereafter, the hot-rolled steel sheet was annealed in 25seconds at 950° C. and rolled to form a cold-rolled steel sheet having athickness of 0.5 mm by cold rolling including pickling. Subsequently,the cold-rolled steel sheet was finish annealed for 20 seconds at 810°C. in various annealing atmospheres and an insulating film was formed.In the finish annealing, a dew point was adjusted in a mixed gasatmosphere containing 35% H₂ and 65% N₂ to prepare various oxygenpotentials P(H₂O)/P(H₂) so that the amount of oxygen on the surfacelayer of the steel sheet was controlled. Thereafter, stress reliefannealing was performed for 2 hours at 750° C. in a dry nitrogenatmosphere, and magnetic properties were measured by the 25 cm Epsteinmethod. Table 3 shows the results of the analysis of components,magnetism measurement and evaluated punching property, respectively.

TABLE 3 Composition Annealing (After (*) (**) tempera- Amount of stressPunching REM Ti Zr N P(H₂O)/ ture oxygen re- property No. Si Al Mn (ppm)(ppm) (ppm) (ppm) P(H₂) (° C.) (g/m²) lief) *** Reference 1 1.19 1.610.31 8 10 25 30 0.002 810 0.02 2.37 ∘ Inventive Example 2 1.19 1.61 0.318 10 25 31 0.05 810 0.09 2.37 ∘ Inventive Example 3 1.19 1.61 0.31 8 1025 30 0.2 810 0.12 2.38 ∘ Inventive Example 4 1.19 1.61 0.31 8 10 25 600.25 810 0.21 2.42 ∘ Inventive Example 5 1.19 1.61 0.31 8 10 25 70 0.5810 0.41 2.44 ∘ Inventive Example 6 1.19 1.61 0.31 8 10 25 33 0.2 8100.2 2.34 ∘ Inventive Example 7 1.21 2.1 0.32 9 12 25 91 0.8 810 1.1 2.88∘ Comparative Example 8 1.19 1.61 0.31 8 10 25 150 0.9 810 1.3 2.91 ∘Comparative Example 9 1.19 1.61 0.31 8 10 25 80 0.4 950 0.6 2.43 ∘Inventive Example 10 1.19 1.61 0.31 8 10 25 130 0.4 1150 1.3 2.71 ∘Comparative Example 11 1.19 1.61 0.31 8 10 25 62 0.4 680 0.2 2.47 ∘Inventive Example 12 1.19 1.61 0.31 8 10 25 30 0.4 550 0.2 2.91 XComparative Example (*) N: Amount of nitriding after stress reliefannealing (**) Amount of oxygen on metal surface layer is changed. ∘:good X: poor

It can be found from Table 3 that when the amount of oxygen on the metalsurface layer of the steel sheet having been subjected to the finishannealing exceeded 1.0 g/m², iron loss was deteriorated. It can beassumed that the deterioration was caused by nitriding in the stressrelief annealing because the amount of N abruptly increased at the time.The amount of oxygen on the metal surface layer could be controlled to apreferable range by regulating the oxygen potentials P(H₂O)/P(H₂) of theatmosphere to 0.7 or less.

Example 4

A slab containing the components shown in Table 4 was made by continuouscasting after it was subjected to a converter-degassing process. Theresulting slab was reheated and then hot rolled to form a hot-rolledsteel sheet. Thereafter, the hot-rolled steel sheet was annealed for 25seconds-5 hours at 950° C. and rolled to form a cold-rolled steel sheethaving a thickness of 0.5 mm by pickling/cold rolling. Subsequently, thecold-rolled steel sheet was finish annealed for 9 seconds or 30 secondsat 800° C. and made into products. In the finish annealing, a dew pointwas adjusted in a mixed gas atmosphere containing 35% H₂ and 65% N₂ toprepare various oxygen potentials P(H₂O)/P(H₂)=0.002 so that the amountof oxygen on the surface layer of the steel sheet was controlled to 0.02g/m². After the components of the resulting products were analyzed andthe inclusions of the products were examined, test pieces were sampledand subjected to stress relief annealing for 2 hours at 750° C. in a drynitrogen atmosphere, and magnetic properties were measured. The resultof the investigation is summarized in Table 4.

TABLE 4 Manufacturing conditions An- neal- ing Properties Compositionfor Finish annealing Amount W_(15/50) (*) hot Tem- Tem- of (after REM TiZr N rolled perat perat oxygen B₅₀ stress No. Si Al Mn (ppm) (ppm) (ppm)(ppm) sheet ure Time ure Time (g/m²) (T) relief) Reference 1 1.19 1.610.31 8 10 25 31 Done 950° C. 25 sec 800° C. 30 0.02 1.74 2.38 Inventivesec Example 2 1.19 1.61 0.31 8 10 25 29 Done 950° C. 1.5 800° C. 30 0.021.75 2.39 Inventive sec Example 3 1.19 1.61 0.31 8 10 25 30 Done 950° C.6.5 800° C. 30 0.02 1.75 2.42 Inventive sec Example 4 1.19 1.61 0.31 810 25 28 Done 830° C. 5 hr 800° C. 30 0.02 1.76 2.3 Inventive secExample 5 1.19 1.61 0.31 8 10 25 32 Not — — 800° C. 30 0.02 1.72 2.45Inventive done sec Example 6 1.19 1.61 0.31 8 10 25 30 Done 950° C. 25sec 800° C. 9 sec 0.02 1.74 2.39 Inventive Example 7 1.2 1.59 0.32 0 1720 30 Done 950° C. 25 sec 800° C. 30 0.02 1.72 2.91 Compara- sec tiveExample 8 1.2 1.59 0.32 0 17 20 31 Done 950° C. 1.5 800° C. 30 0.02 1.712.9 Compara- sec tive Example 9 1.2 1.59 0.32 0 17 20 32 Done 950° C.6.5 800° C. 30 0.02 1.71 2.89 Compara- sec tive Example 10 1.2 1.59 0.320 17 20 29 Done 830° C. 5 hr 800° C. 30 0.02 1.73 2.78 Compara- sec tiveExample 11 1.2 1.59 0.32 0 17 20 28 Not — — 800° C. 30 0.02 1.7 2.99Compara- sec tive done Example 12 1.2 1.59 0.32 0 17 20 29 Done 950° C.25 sec 800° C. 9 sec 0.02 1.74 2.92 Compara- tive Example (*) N: Amountof nitriding after stress relief

It is found from Table 4 that since the inventive examples to which REMwas added and in which the amounts of Ti and Zr were reduced wereexcellent in stress relief annealing, the properties thereof were notdeteriorated even if the hot-rolled steel sheet was annealed for arelatively short time of 40 seconds or less at 950° C., and theproducts, which were excellent particularly in magnetic flux density ascompared with the products to which no REM was added, could be obtained.Furthermore, even if the annealing time was set to 30 seconds and 9seconds, there was no difference between the grain growing properties.Therefore, it is possible to perform the finish annealing for a shorttime of 15 seconds or less which is not conventionally employed.

It was discovered that the hot-rolled steel sheet annealing time and thefinish annealing time could be reduced by the improvement of the graingrowing property which was achieved by controlling the amounts of REM,Ti, Zr and the like. As a result, there is a large possibility thatproductivity can be greatly improved by the present invention.

Since the non-oriented electromagnetic steel sheet provided by thepresent invention includes preferable mechanical properties realized bythe increase of Si and Al, the high magnetic flux density can bemaintained without sacrificing the punching property as well as the verylow iron loss can be obtained even after stress relief annealing.Accordingly, the non-oriented electromagnetic steel sheet of the presentinvention is ideal as a material for small high-efficiency motorssuitable for use in household electrical appliances and the like.

What is claimed is:
 1. A finish annealed non-oriented electromagneticsteel sheet, comprising about 0.01 wt % or less of C, greater than about1.0 wt % and at most about 3.5 wt % of Si, at least about 0.6 wt % andat most about 3.0 wt % of Al, at least about 0.1 wt % and at most about2.0 wt % of Mn, at least about 2 ppm and at most about 80 ppm of one ormore rare earth metals (REM), a maximum content of Ti and Zr being about15 ppm and 80 ppm, respectively, wherein oxygen on the surface layer ofthe steel sheet is 1.0 g/m² or less.
 2. The non-oriented electromagneticsteel sheet according to claim 1, further comprising at least about0.002 wt % and at most about 0.1 wt % of at least one of Sb and Sn. 3.The non-oriented electromagnetic steel sheet according to claim 1,further comprising S, O and N in amounts of 20 ppm or less, 15 ppm orless and 30 ppm or less, respectively, wherein the ratio ofREM-containing inclusions coupled with nitride to REM-containinginclusions having a diameter of at least about 1 μm in the steel sheetis 40% or more.
 4. The non-oriented electromagnetic steel sheetaccording to claim 2, further comprising S, O and N in amounts of 20 ppmor less, 15 ppm or less and 30 ppm or less, respectively, wherein theratio of REM-containing inclusions coupled with nitride toREM-containing inclusions having a diameter of at least about 1 μm inthe steel sheet is 40% or more.